The invention relates to detecting a change in the polarity of the slope of an electrical signal.
The analysis of many physical processes often requires determination of the slope of an electrical signal that represents an aspect of the process. In gas chromotography a gas containing molecules of different sizes is injected into one end of a long, dust-filled cavity. The gas molecules travel through the cavity toward the other end where a detector is located. During transit the larger molecules collide with a greater number of dust particles than the smaller molecules, and the smaller molecules thus reach the detector first. The dust chamber causes molecules of the same size to travel together in groups, the group composed of the smallest molecules arriving at the detector first, followed by the next larger molecules, until finally the largest molecules reach the detector. The detector measures the number of molecules arriving at each instant of time and produces a continuous electrical signal having a time dependent series of peaks and valleys. The peaks represent the groups of molecules of the various sizes, and the lowest points in the valleys indicate the end of one group of molecules and the beginning of a group of larger molecules. At such a valley in the signal, the slope changes from negative to positive. Thus, the beginning of a new group of molecules whose size is larger than the preceding group can be determined by detecting a change in a signal slope from positive to negative.
One common circuit used to detect changes in slope is described in the ELECTRONIC CIRCUIT MANUAL By John Marcus (McGraw-Hill, 1971), at page 202. The detector has as its primary element a differential amplifier having a positive and a negative input. The negative input connects directly to the signal being analyzed, while the positive input communicates with the signal through a delay network. The delay network comprises a resistor and a capacitor. According to well-known principles, the signal entering the positive input will thus be delayed by an amount proportional to the product of the value of the resistor and the value of the capacitor. The differential amplifier thus sees at each input the signal being analyzed, but the signal to the positive input is delayed by a small amount of time. Thus, the signal level at the positive input will lag behind the level at the negative input. The result will be a voltage differential between the two inputs that will be amplified and appear at the output of the differential amplifier. The output will be a two-level signal which exhibits a positive voltage level when the signal at the negative input of a differential amplifier is larger than that at the positive and a negative voltage when the signal at the positive input is larger than that at the negative. When the two signals are equal, there will be a zero output voltage level. The time at which the two input levels are equal represents the point in the signal at which the slope changes. For example, assuming an input signal that is initially increasing and thus has a positive slope, the positive input of a differential amplifier will be lower than the negative input until the signal reaches a peak and begins to decrease in level. Thus, while the positive input is still increasing the negative input will be decreasing, and after the delay period of the lag network the two input signals will be equal.
Due to the nature of the delay method of detecting a change in slope, the point at which the output of the differential amplifier reaches the zero voltage level, will follow the actual change in slope by an amount of time imposed by the delay network. The delay in the prior art circuit is unacceptably large for some applications, resulting in indications of slope changes being given long after the changes have occurred. It is thus desirable to have a delay network in a slope change detector that is capable of producing a shorter and much more precisely controlled delay.
It is therefore an object of the present invention to provide a slope change detector having the above characteristics.